Second Class Condition - Is known as the 'Contrary-to-Fact Condition' and assumes the premise as false for the sake of argument. The protasis is again formed with the helping word ei ('if') and the main verb in the indicative mood. The tense of the verb (in the protasis) must also be in a past-time tense (aorist or imperfect). The apodosis will usually have the particle an as a marking word, showing some contingency. [Link: http://www.ntgreek.org/learn_nt_greek/c ... tences.htm.]
As Smyth says, all we need is εἰ + the aorist indicative in the protasis, ἄν + the aorist indicative in the apodosis. This condition is exactly the kind we find at one point in Mark's apocalyptic discourse:
Notice that verse 19 is a prediction of the future: there will be a tribulation period such as has never happened before. It references the past in order to clarify that the future will be far worse than anything in the past could have prepared us for; but the prediction itself is in the future tense, as we ought to expect. The prediction itself is based on a prophecy from Daniel:
But what about verse 20? There is nothing futuristic about it at all. The tenses are aorist (the normal Greek "past tense" in the indicative mood, which is what we have here). The conditional statement is a straight-up, nothing-fancy past contrary-to-fact condition, no different than saying in English, for example, "If you had invited me to your party (but you did not), I would have brought you a present (which I did not)." In this case: "If the Lord had not shortened those days (but he did), nobody would have survived (which they did)."
The question presses: why, if the days of tribulation are in the future tense in verse 19, are they now suddenly in the past tense in verse 20?
I believe a simple answer may suggest itself. Verse 19 is an actual prediction; but verse 20 is a gloss on that prediction. Somebody predicted that the greatest tribulation of all time, past or future, was going to come soon, and that prediction was partially fulfilled, but not entirely. Either the tribulation was not the worst of all time or, worse yet, it did not immediately lead into the other events that even a cursory reading of Daniel 12.1-3 would suggest, such as the regathering of Israel and resurrection of the dead. So what had happened? Obviously, the prediction was fulfilled: there was a period of great tribulation, but God had mercifully cut short the days, thus creating a span of time between that tribulation and anything else that had been promised to come:
Ben.
ETA:
Here are some examples of textbook past contrary-to-fact conditions from the Greek NT (besides Matthew 24.22 = Mark 13.20):
Matthew 11.23: 23 "And you, Capernaum, will not be exalted to heaven, will you? You shall descend to Hades; for if the miracles had occurred in Sodom which occurred in you, it would have remained to this day [ὅτι εἰ ἐν Σοδόμοις ἐγενήθησαν αἱ δυνάμεις αἱ γενόμεναι ἐν σοί, ἔμεινεν ἂν μέχρι τῆς σήμερον]."
Luke 10.13: 13 "Woe to you, Chorazin! Woe to you, Bethsaida! For if the miracles had been performed in Tyre and Sidon which occurred in you, they would have repented long ago, sitting in sackcloth and ashes [ὅτι εἰ ἐν Τύρῳ καὶ Σιδῶνι ἐγενήθησαν αἱ δυνάμεις αἱ γενόμεναι ἐν ὑμῖν, πάλαι ἂν ἐν σάκκῳ καὶ σποδῷ καθήμενοι μετενόησαν]."
Romans 9.29: 29 And just as Isaiah foretold, "If the Lord of Sabaoth had not left to us a posterity, we would have become as Sodom, and would have resembled Gomorrah [εἰ μὴ κύριος σαβαὼθ ἐγκατέλιπεν ἡμῖν σπέρμα, ὡς Σόδομα ἂν ἐγενήθημεν καὶ ὡς Γόμορρα ἂν ὡμοιώθημεν]."
1 Corinthians 2.8: 8 ...the wisdom which none of the rulers of this age has understood; for if they had understood it, they would not have crucified the Lord of glory [εἰ γὰρ ἔγνωσαν, οὐκ ἂν τὸν κύριον τῆς δόξης ἐσταύρωσαν]....